improve parameter template docs

[oliver-sanders]

We can template parameters into environment variables, e.g:

[runtime]
  [[mytask<param>]]
    [[[environment]]]
      foo = %(param)s

This gets touched on in three places:

• https://cylc.github.io/cylc-doc/stable/html/user-guide/writing-workflows/parameterized-tasks.html#parameter-expansion
• https://cylc.github.io/cylc-doc/stable/html/reference/config/workflow.html#flow.cylc[runtime][%3Cnamespace%3E][environment]%3Cvariable%3E
• https://cylc.github.io/cylc-doc/stable/html/reference/config/workflow.html#flow.cylc[task%20parameters][templates]%3Cparameter%3E

But isn't really clearly explained in any of these locations.

Additionally, the tutorial documents the (older) CYLC_TASK_PARAM syntax, we should prefer the newer template syntax:

• https://cylc.github.io/cylc-doc/stable/html/tutorial/runtime/configuration-consolidation/parameters.html#environment-variables